qiskit/releasenotes/notes/1.2/sabre_level0-1524f01965257f...

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---
upgrade_transpiler:
- |
The default routing pass used by optimization level 0 for :func:`.generate_preset_pass_manager`
and :func:`.transpile` has been changed from :class:`.StochasticSwap` to :class:`.SabreSwap`.
The :class:`.SabreSwap` pass performs exactly the same function but performs better in both
runtime and output quality (in number of swap gates and depth) compared to
:class:`.StochasticSwap`. For ``optimization_level=0`` this shouldn't matter because it's not
expected to run routing for the typical use case of level 0.
If you were relying on the previous default routing algorithm for any reason you can use the
``routing_method`` argument for :func:`.transpile` and :func:`.generate_preset_pass_manager`
to ``"stochastic"`` to use the :class:`.StochasticSwap` pass.